Microorganisms such as algae, bacteria or fungi may contain triglycerides up to 80% of their total dry matter content. However, oil from microbial biomass which is suitable as precursor for fuel production is scarce on the market. This is mainly due to lack of efficient and economical methods for providing good quality oil from microbial biomass.
The available methods for extracting oil or lipids from microbial biomass typically require the biomass to be dried and/or the microbial cells to be disrupted. Drying of the biomass consumes much energy and is, for example, performed by heating or freeze-drying, or even spray drying is used. The typical water content or the dry matter content of the biomass is dependent on the microbial material used. Typically dry matter contents from 15% up to 40% can be achieved by traditional cell harvesting techniques such as centrifugation or filtration. Essentially, it is typically aimed at as low free water content as possible in order to maximize the extraction yields.
One alternative method to acquire oil from biomass is to apply non-selective extractants which typically produces oil containing high amounts of impurities. Impurities such as metals, phosphorus and amino acids cause problems e.g. in catalytic fuel production in form of catalyst poisons and/or corrosive materials. Therefore, it is often required to use post processing for removal of these undesired components from the extracted oil product.
In general, methods available suffer either from lack of selectivity to produce good quality oil or poor yield which are compensated by additional processing steps or selection of uneconomical processing conditions.
One of the commonly used extractants for retrieval of oils from oil containing materials is carbon dioxide in subcritical or preferably supercritical state. Unfortunately carbon dioxide has a weak dissolving capacity towards vegetable neutral lipids. This property has been modified by incorporation of entraining agents exhibiting better dissolution such as propane or butane. Supercritical fluids have better dissolution capacities but require high operating pressures. Moreover, the extraction times also tend to remain uneconomically long.
CA-2165387 discloses a selective extraction of fats and/or oils from solid natural materials, such as microbial solids, like dried and pelletized fermentation residues having water content less than 5%, with compressed gases. The extraction is carried out using a mixture of propane and maximum of 50% by weight of carbon dioxide at temperatures below 96° C. and pressures below 73 bar. These two pure gases each are in the subcritical state. In this method an energy consuming drying step prior to the extraction is carried out.
U.S. Pat. No. 4,331,695 discloses a method for extracting fats or oils from animal or vegetable products, such as soya flakes or maize, by contacting the product with a hydrocarbon solvent, such as propane, in the liquid phase and at a temperature below the critical temperature, separating the solvent containing extracted fat or oil from the residue of the product, and precipitating the extracted fat or oil from the solvent by heating the solvent to above the critical temperature of the solvent without taking up heat of vaporization. The resulting fat or oil is suitable for use in foodstuffs without further processing to remove solvent. The temperatures of propane may be 0-100° C. during extraction and 50-200° C. during precipitation of oil and fat, and the pressure may be the same during extraction and precipitation. The disclosed method does not disclose any pretreatment of the material to be extracted and the method does not include extraction of microbial biomass.
Occasionally, when extracting oil from microbial biomass mechanical disruption, dissemination or crushing of biomass cells has been used or even microwave assisted disruption of biomass cells to aid in increasing the extracted oil yield.
US-2003/0143659 discloses a process for isolation of compounds such as polyunsaturated fatty acids from micro-organisms, in which process the biomass is first granulated and subsequently dried before extraction of the biomass. The biomass can be pasteurised in the growth medium, i.e. heated to 60-100° C., typically preferably to 96° C., up to 90 minutes in order to kill the micro-organism and also to deactivate enzymes which might otherwise destroy the fatty acids. The pasteurisation has no reported effect on the subsequent extraction.
WO2006136539 discloses an extraction process where lipids are obtained from wet biomass. In this process a desiccant is added to the wet biomass prior to extration to remove excess water. The biomass is pasteurised at 65° C. for 1 h prior to the extraction process.